This invention relates to transmission lines and the termination thereof.
In particular the present invention is concerned with the termination of high speed radio frequency (RF) transmission line electrode structures of various forms such as microstrip, co-planar or other alternative geometries.
A factor of considerable importance in the termination of transmission lines is to ensure that any signal reflection of the radio frequency signal arising from the termination should be as low as possible. With such an object in view various proposals have been made in relation to termination of high frequency radio signals.
In practice, it is considered that as the frequency of operation of a transmission line increases the physical size the he termination structure decreases due to the consequential reduction in the signal wavelengths involved.
It is known in the construction of transmission line terminations to make use of a relatively wide variety of materials such as for example, insulating substrates such as ceramics, quartz, or circuit boards from organic materials. Conventionally metallic conductor patterns are applied to the insulating substrates in such arrangement as to form planar transmission lines used for routing either the radio frequency or high-speed digital signals.
In practice, in the formation of both co-planar and microstrip waveguides, the choice of the substrate materials and the particular transmission line structure used is highly dependent on the specific application for the resulting waveguide. In addition, the choice of substrate and transmission line structure influences the performance, size, and cost of the assembled waveguide. However, in general the microstrip design is more widely used than the co-planar design due to its structural robustness and its compatibility in interfacing with active devices. One particular attractive aspect of the known designs of microstrip waveguides is that for a fixed substrate thickness, as the relative dielectric constant, ∈r, of the substrate is increased the circuit size is decreased. In view of this alumina (Al2O3) is a common choice from possible substrate materials as it has a relative dielectric constant ∈r≈10 and so allows compact microelectronic packages to be manufactured.
However, when designing a microstrip circuit of alumina or other high dielectric constant substrate where there is a need for a serial broadband DC block in a compact circuit layout a design conflict arises.
The serial capacitor assembly is physically large and the requisite mounting pad size is larger than the transmission line width required for 50Ω characteristic impedance. The additional shunt capacitance of the mounting pad results in a poor impedance match to the transmission line. This mismatch in impedance will result in a proportion of the RF signal being reflected rather than transmitted.
It has been found that increasing the substrate thickness will also have the effect of degrading the broadband termination performance in a microstrip design due to the increased path length of the connection that is necessary between the top and bottom surfaces of the substrate resulting in an additional serial inductance.
Another problem with using conventional microstrip or co-planar structures is the difficulty in moving from one form of structure to another when using the same substrate. Thus when effecting a transition from a conventional microstrip waveguide to a conventional co-planar waveguide using the same substrate difficulties arise.
In a conventional co-planar waveguide structure the signal is applied to a central conductor and coupled to two relatively wide ground conductors located on either side of the central conductor on the same side of the substrate. As a result in effecting the transition from the microstrip waveguide there is a need to connect the microstrip ground conductor with the ground conductors of the co-planar waveguide, which are conventionally located on the opposite side of the substrate as compared with that of the microstrip waveguide. It has been found that the effecting of this required interconnection of the ground planes inherently adversely affects the high frequency operation of the combined structure.